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With the aim to improve the design of metal complexes as stabilizers of noncanonical DNA secondary structures, namely, G-quadruplexes (G4s), a series of cyclic dinuclear Au(I) N-heterocyclic carbene complexes based on xanthine and benzimidazole ligands has been synthesized and characterized by various methods, including X-ray diffraction. Fluorescence resonance energy transfer (FRET) and CD DNA melting assays unraveled the compounds' stabilization properties toward G4s of different topologies of physiological relevance. Initial structure-activity relationships have been identified and recognize the family of xanthine derivatives as those more selective toward G4s versus duplex DNA. The binding modes and free-energy landscape of the most active xanthine derivative (featuring a propyl linker) with the promoter sequence cKIT1 have been studied by metadynamics. The atomistic simulations evidenced that the Au(I) compound interacts noncovalently with the top G4 tetrad. The theoretical results on the Au(I) complex/DNA Gibbs free energy of binding were experimentally validated by FRET DNA melting assays. The compounds have also been tested for their antiproliferative properties in human cancer cells in vitro, showing generally moderate activity. This study provides further insights into the biological activity of Au(I) organometallics acting via noncovalent interactions and underlines their promise for tunable targeted applications by appropriate chemical modifications.

We report here on the synthesis of a series of mono- and dinuclear gold(I) complexes exhibiting sulfonated bis(NHC) ligands and novel hydroxylated mono(NHC) Au(I) compounds, which were also examined for their biological activities. Initial cell viability assays show strong antiproliferative activities of the hydroxylated mono(NHC) gold compounds (8 > 9 > 10) against 2008 human ovarian cancer cells even after 1 h incubation. In order to gain insight into the mechanism of biological action of the gold compounds, their effect on the pivotal cellular target seleno-enzyme thioredoxin reductase (TrxR), involved in the maintenance of intracellular redox balance, was investigated in depth. The compounds' inhibitory effects on TrxR and glutathione reductase (GR) were studied comparatively, using either the pure proteins or cancer cell extracts. The results show a strong and selective inhibitory effect of TrxR, specifically for the hydroxyl-functionalized NHC gold(I) complexes (8-10). Valuable information on the gold compounds' molecular reactivity with TrxR was gained using the BIAM (biotin-conjugated iodoacetamide) assay and performing competition experiments by mass spectrometry (MS). In good agreement, both techniques suggest the binding affinity of the mono(NHC) Au(I) complexes toward selenols and thiols. Notably, for the first time, bis-carbene formation from mono-carbenes in buffered solution could be observed by MS, which may provide new insights into the speciation mechanisms of bioactive Au(I) NHC complexes. Furthermore, the compounds' interactions with another relevant in cellulo target, namely telomeric G-quadruplex DNA-a higher-order DNA structure playing key roles in telomere function-was investigated by means of FRET melting assays. The lack of interactions with this type of nucleic acid secondary structure support the idea of selective targeting of the hydrophilic Au(I) NHC compounds toward proteins such as TrxR.